1. Field of the Invention
This invention relates to zoom lenses and, more particularly, to zoom lenses of the so-called "inner" or "rear" focus type suited to be used in photographic cameras, video cameras or broadcasting cameras. Still more particularly, this invention relates to the structure of a cam for moving either a lens unit behind the zooming section or one of the lens units of the zooming section to effect focusing, while keeping the image always at a good quality.
2. Description of Related Art
For the zoom lenses to be used in cameras or the like, there have been many previous proposals on the type that moves the lens unit for focusing forward by varying amounts with variation of the zooming position, or the so-called "inner" focus type (or "rear" focus type).
In general, the use of the inner focus type produces outstanding advantages over that type of zoom lens which uses the first lens unit in focusing. Of these, the first lens unit becomes smaller in the effective diameter. This facilitates improvements of the compact form of the entirety of the lens system. Also, close-up photography, particularly, supershort focusing, becomes easier to perform. Further, because the lens unit to be used is relatively small in size and light in weight, a weaker driving force suffices for moving that lens unit. This leads to the possibility of making rapid focus adjustment.
With the zoom lens of the inner focus type, during zooming (or varying the focal length) from the wide-angle end to the telephoto end, the focusing movement has to be varied despite an object remaining at the same distance. For this reason, a special cam mechanism is used to make the control.
Taking an example of the zoom lens comprising, in order from an object side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, the first, second and fourth lens units are moved to effect zooming and the third lens unit is moved to effect focusing. In this inner focus type zoom lens, despite the object remaining at the same distance, the third lens unit for focusing has to change its axial position as the focal length setting changes. Because of this, the operating mechanism for such a zoom lens detects the axial position of the lens unit for varying the focal length and specifies the current zooming position. By this, the focusing lens unit is moved to an adjusted position.
In many prior known zoom lenses, the focusing lens unit is brought to operative connection with another movable lens unit or units, thus reducing the number of cams.
FIG. 3 is a longitudinal section view of a mechanical mounting for such a zoom lens. FIG. 4 is a schematic diagram of the loci of motion with zooming of all the lens units of FIG. 3.
In FIG. 3, the first lens unit 1, the second lens unit 2 and the fourth lens unit 4 have cam followers or pins 11, 12 and 14, respectively. The third lens unit 3 is used for focusing. A helicoid bar 13 extends axially from the holder of the second lens unit 2. A cam ring 15 has cam grooves. A fixed tube 16 has an axially elongated guide slot. When the cam ring 15 is rotated with respect to the fixed tube 16, the first lens unit 1, the second lens unit 2 and the fourth lens unit 4 are moved forward in differential relation as determined by their cams, thus varying the focal length. During this time, the cam pins 11, 12 and 14 are restrained from moving around an optical axis L. Therefore, the first lens unit 1, the second lens unit 2 and the fourth lens unit 4 do not rotate when moving axially forward.
Meanwhile, the third lens unit 3 is moved along the optical axis L by the helicoid bar 13 extending from the second lens unit 2, so that the relative position of the third lens unit 3 to the second lens unit 2 is varied. Although the third lens unit 3 is linked to the second lens unit 2, the zooming movements of the two lens units 2 and 3 are, therefore, caused to differ from each other. That is, the second and third lens units move also in differential relation. Further, the third lens unit 3, even during focusing, moves along the optical axis L although the second lens unit 2 remains stationary. That is, the third lens unit 3 moves not only during zooming, but also during focusing, playing roles like the so-called compensator which also serves as the focusing lens. Such a third lens unit 3 is made movable under the control of what is called an "electronic cam".
In general, the use of the inner focusing method in the zoom lens leads, as described before, to obtain the advantages of improving the compact form of the entire lens system, making it possible to do rapid focusing, and decreasing the minimum object distance.
However, on the other hand, the axial position of the lens unit for varying the focal length must be detected with very high accuracy in determining the zooming position. Otherwise, the lens unit for focusing could not be put at a right axial position, and a focusing error would result. FIG. 4 is a diagram for explaining the paraxial power arrangement of the zoom lens of FIG. 3. The arrows show the directions of movement of the lens units during zooming from the wide-angle end W to the telephoto end T. Incidentally, the illustrated zoom loci are the ones which occur when the zoom lens is focused on an object at infinity.
Referring to FIG. 4, assume that the zooming position of the zoom lens is actually in the telephoto end T. Further, suppose, at this time, a detecting means for detecting the zooming position has made such error that the zooming position of the zoom lens is determined to be in a zooming position X. In this case, the separation between the second lens unit 2 and the third lens unit 3 should, as a rule, take a distance DT for the telephoto end T. Nonetheless, a distance DX for the zooming position X is adopted in controlling the separation between the second lens unit 2 and the third lens unit 3. Under this condition, therefore, the position of the third lens unit 3 is adjusted not to a point "b" but to a point "a". Hence, a near focus state results as the third lens unit 3 has been moved from the point "b" toward the object side in excess by a distance (DT-DX).
Again, conversely assume that although the actual zooming position is in the zooming position X in FIG. 4, the detecting means for the zooming position has determined that the zooming position is in the telephoto end T. In this case, the separation between the second lens unit 2 and the third lens unit 3 should be the distance DX. Nonetheless, it happens that the third lens unit 3 is moved over the point "a" down to the point "b". This means that the third lens unit 3 moves too longer rearward by the distance (DT-DX). Hence, a far focus state results as is opposite to the former.
In recent years, zoom lenses of ever improved compact form are to be developed. To this purpose, the refractive power of each lens unit becomes stronger. So, the lens sensitivity tends to rise. Particularly for the focusing lens unit, the tolerance to specify the sharp focus is severe. In this case, the reading error of the zooming position, however slight it may be, will result in so much a large deviation from the sharp focus as is not negligible.